This invention relates to structured phosphor screens.
Phosphor screens are used in the field of x-ray detection and imaging to convert high-energy x-ray photons into lower-energy optical photons; for each incident x-ray photon, multiple optical photons are generated. After passing through the phosphor screen, both x-ray and visible photons can then be detected using devices sensitive to electromagnetic radiation, such as an emulsified film or CCD camera.
In general, x-rays propagating through solid materials may be deflected, scattered, or absorbed. In the case of absorption, fluorescent or phosphorescent materials contained in phosphor screens emit photons in response to incident x-rays, yielding a scintillation of light in the ultra-violet (UV), visible, or infrared (IR) spectral range. In a particular example, calcium tungstate, a frequently used phosphorescent material, emits multiple photons in the spectral region of 400-500 nm following absorption of a single x-ray photon having an energy of about 35 KeV; in this case, 1000 initial x-ray photons result in the generation of about 180,000 optical photons (Physics of Radiology, A. B. Wolbarst, p. 184, 1993). The emission process, therefore, effectively amplifies the number of photons available for detection, thereby allowing high-contrast images to be produced.
In medical imaging applications, phosphor screens may be incorporated into radiographic cassettes, where they are used to expose a sheet of emulsified film. Typically, the phosphorescent material is polycrystalline in form and embedded in a binder; individual crystals of the phosphorescent material are usually on the order of 10 .mu.m in diameter. Following assembly, the radiographic cassette is positioned relative to a patient and x-ray source in order to detect x-rays passing through a plane of a target volume (such as a region of tumorous tissue) in the patient. To expose the film, an incident x-ray photon is absorbed by the phosphor screen, resulting in the emission of a series of lower-energy photons; through this process, the details of the target volume are imaged onto the film.
Alternatively, electronic detection devices, such as CCD cameras or diode arrays, can be placed behind the exposed phosphor screen to allow for detection of the emitted photons. Such equipment can be used to provide real-time imaging of objects in rapid motion, such as the heart.
In all cases, it is desirable to reduce the patient's risk of x-ray exposure during imaging procedures. One approach to minimize the amount of x-ray radiation required to produce a given image is to increase the amount of photon amplification (i.e., the degree of conversion of x-rays to optical photons) produced by the phosphor screen. This can be done using a number of methods: screens can be made thick (e.g., between 150 .mu.m and 1 mm) to increase the probability that incident x-ray photons are absorbed by the fluorescent or phosphorescent materials; alternatively, the density of such materials in the screen can be increased to improve the probability that x-ray absorption occurs.
While both of these methods increase the degree of photon amplification produced by the phosphor screen, the resolution, or sharpness of the resultant image, typically decreases correspondingly. Optical photons emitted following x-ray absorption propagate outwardly and randomly in all directions. Spatial dispersion of the emitted optical field, as well as diffusion and scattering off of neighboring crystalline material in the phosphor screen, enlarges the area of the film (or detector) that is illuminated, resulting in a blurring of the resultant image. Additionally, the intensity of the emitted light decreases as it propagates away from the point of x-ray absorption, resulting in a further decline of image quality.
In summary, there exists a reciprocal relationship between the degree of amplification and resolution provided by the phosphor screen. Thicker screens provide increased sensitivity and decreased resolution for a given x-ray level. Thinner screens result in a sharper x-ray image, but require more x-ray photons to produce the image, resulting in an increase in the x-ray dosage to which the patient is exposed.
One technique used to fabricate high-resolution phosphor screens which provide suitable x-ray sensitivity involves segmenting the phosphor screen into a two-dimensional array of pixels. Following x-ray absorption, this design allows each pixel to channel emitted radiation to the light-sensitive detector. If the pixels are optically isolated, spatial dispersion and scattering effects which normally blur the x-ray-induced image are reduced. Ito et al., IEEE Trans. Nuc. Sci. Vol. 34, p. 401, describe a two-dimensional phosphor screen which consists of individual CsI(Na) crystals grown on glass fibers etched onto a glass plate. In U.S. Pat. No. 5,302,423, a method for fabricating pixelized phosphors using optical ablation techniques is described.